record rational(numer, denom, sign)        # rational type

   addrat(r1,r2) # Add rational numbers r1 and r2.
   divrat(r1,r2) # Divide rational numbers r1 and r2.
   medrat(r1,r2) # Form mediant of r1 and r2.
   mpyrat(r1,r2) # Multiply rational numbers r1 and r2.
   negrat(r)     # Produce negative of rational number r.
   rat2real(r)   # Produce floating-point approximation of r
   rat2str(r)    # Convert the rational number r to its string representation.
   real2rat(v,p) # Convert real to rational with precision p (default 1e-10). Warning: excessive p gives ugly fractions
   reciprat(r)   # Produce the reciprocal of rational number r.
   str2rat(s)    # Convert the string representation (such as "3/2") to a rational number
   subrat(r1,r2) # Subtract rational numbers r1 and r2.

   gcd(i, j)     # returns greatest common divisor of i and j